1. Technical Field of the Invention
The present invention relates to a magnetoresistive effect element in a current perpendicular to plane (CPP) structure that includes two free layers for reading, as a signal, a magnetic field intensity of a magnetic recording medium and the like, a thin film magnetic head having the magnetoresistive effect element, and a head gimbal assembly and a magnetic disk device including the thin film magnetic head.
2. Description of Conventional Art
With the recent increase in a recording density of hard disk drive (HDD), enhancement of performance of the thin film magnetic head has been desired. A composite thin film magnetic head, in which a magnetoresistive effect element (hereinafter may be referred to simply as an MR element) exclusively for reading out signals and an inductive magnetic transducer exclusively for writing signals are laminated, has been widely used as the thin film magnetic head.
Recently, a so-called magnetoresistive effect element in a current in plane (CIP) structure that operates by passing an electric current in parallel with a film surface of an element called a spin-valve giant magnetoresistive effect (GMR) element (CIP-GMR element) is widely used as a reproducing head. The spin-valve GMR element with such a structure is positioned between upper and lower shield layers formed by soft magnetic metal films and arranged to be sandwiched by insulation layers called gap layers. The recording density in a bit direction is determined by a gap between the upper and lower shield layers (length of a shield gap or a read gap).
With an increase in the recording density, needs for narrow shield gaps and/or narrow tracks have been increased for the reproducing element for the reproducing heads. However, element areas are reduced due to the narrowing of the tracks of the reproducing element and the shortening of a height of the reproducing element caused by the narrowing of the tracks. Therefore, there are problems with the conventional structure that the heat dissipation efficiency is decreased by the reduction of the areas, and that the operating current is limited from a viewpoint of reliability.
To solve such problems, a CPP-GMR element that electrically serially connects the upper and lower shield layers and the MR element and that does not require an insulation layer between the shield layers has been proposed. This technology is considered necessary for achieving a recording density that exceeds 200 Gbits/in2.
The CPP-GMR element has a layered structure that includes first and second ferromagnetic layers formed to sandwich a conductive non-magnetic intermediate layer at both sides. The layered structure of a well-known spin-valve type CPP-GMR element sequentially includes, from a substrate side, a lower electrode, an antiferromagnetic layer, a first ferromagnetic layer, a conductive non-magnetic intermediate layer, a second ferromagnetic layer and an upper electrode.
A magnetization direction of the first ferromagnetic layer, which is one of the two ferromagnetic layers, is pinned to be perpendicular with a magnetization direction of the second ferromagnetic layer when there is no externally applied magnetic field. The magnetization direction of the first ferromagnetic layer is pinned by arranging the antiferromagnetic layer adjacently to the first ferromagnetic layer and by applying a unidirectional anisotropic energy to the first ferromagnetic layer due to an exchange-coupling of the antiferromagnetic layer and the first ferromagnetic layer. The unidirectional anisotropic energy may be referred to as an “exchange bias” or a “coupling magnetic field.” Therefore, the first ferromagnetic layer is called a magnetization pinned layer. In contrast, the second ferromagnetic layer is called a free layer. Moreover, by forming the magnetization pinned layer (first ferromagnetic layer) in a three layer structure of a ferromagnetic layer, a non-magnetic metal layer and a ferromagnetic layer (so-called “synthetic ferromagnetic structure” or “synthetic pinned structure”), a strong exchange-coupling is applied between the two ferromagnetic layers, and an exchange-coupling force from the antiferromagnetic layer can be effectively increased. In addition, an influence of a static magnetic field generated from the magnetization pinned layer on the free layer can be reduced. Thus, the “synthetic pinned structure” is currently widely used.
However, to respond to the recent needs for a super-high recording density, further thinning of layers of the MR element is required. U.S. Pat. No. 7,019,371B2, U.S. Pat. No. 7,035,062B1 and U.S. Pat. No. 7,177,122B2, for example, propose a new GMR element structure having a simple three-layer structure, as a base structure, including a ferromagnetic layer (free layer), a non-magnetic intermediate layer, and a ferromagnetic layer (free layer).
Such a structure may be called a dual free layer (DFL) element structure in this application, for convenience. In the DFL element structure, the two ferromagnetic layers (free layers) are exchange-coupled so that their magnetization becomes antiparallel to each other. In addition, as a hard magnet is positioned at a deep position, which is opposite from an air bearing surface (ABS) that corresponds to a medium-opposing surface of the element, an initial state of the element is created, where, using effects of a bias magnetic field generated from the hard magnet, magnetization directions of two magnetic layers (free layers) are each inclined towards a track width direction by approximately 45 degrees.
When the element in this initial magnetization state detects a signal magnetic field from the medium, the magnetization directions of the two magnetic layers change like a motion of a pair of scissors cutting paper. As a result, a resistance value of the element changes.
If this DFL element structure is used in a so-called TMR element or CPP-GMR element, the “read gap length,” which is a gap between the upper and lower shield layers, can be remarkably narrowed compared with a conventional generic spin-valve type CPP-GMR element. More specifically, the above-discussed antiferromagnetic layer, which is required in a generic spin-valve type CPP-GMR element, becomes unnecessary. Further, the ferromagnetic layer with the above-discussed “synthetic pinned structure” becomes unnecessary. As a result, the “read gap length,” which was conventionally said to be limited to 30 nm, can be further shortened.
In the DFL element structure, the read gap length can be shortened, and the recording density in the track direction can be increased.
For the DFL element structure, some of the inventors of the present application have proposed a technique to obtain a very narrow read gap by magnetically coupling two exchange-coupling shield layers, which are made in a single domain by the antiferromagnetic layers, and two free layers in order to control the magnetic direction of the two free layers, and by operating the exchange-coupling shield layers as a shielding (e.g., US2010/0039734).
As is apparent from a schematic diagram of a conventional DFL element structure viewed from the ABS side shown in FIG. 18, non-magnetic metal filled layers 603 formed of a Cr or CrTi layer are formed at both ends of the sensor film 600 including two free layers 630 and 650 that are separated by a non-magnetic layer 640. The sensor film 600 and the non-magnetic metal filled layers 603 are separated by an insulation layer 602 made of Al2O3. As shown in FIG. 19 (FIG. 19 is a cross sectional view seen from an arrow α1-α1 in FIG. 18), on the rear side of these layers, a bias layer 800 for making magnetic directions of the two free layers 630 and 650 to perpendicularly intersect with each other is formed with an insulation layer 710 made of Al2O3 through the bias layer 800. The bias layer 800 may be formed by forming a CoPt layer, as a hard magnet layer, on a Cr base layer.
However, after exhaustive research on the configuration of the bias layer after film formation by the inventors of this application, it was found that an ideal flat bias layer 800 extending in the X direction is essentially difficult to obtain if the non-magnetic metal filled layer 603 is configured simply by a single layer of Cr or CrTi.
That is, as shown in FIG. 20 (FIG. 20 is an overall view of the bias layer 800 from the rear side viewed from the arrow α2-α2 in FIG. 19), the bias layer 800 is not flat in the X direction but includes a U-shaped formation 800a, in which a part of the hard magnet layer positioned at the DFL element part is lowered.
The reason is explained below based on the film formation process for the element configuration. To ease the explanation, the film structure is simplified.
First, as shown in FIGS. 21A and 21B (FIG. 21B is a cross sectional view seen from the arrow α3-α3 in FIG. 21A), a sensor film 600 (or MR film 600) that includes DFL (dual free layer) is formed on a shield layer 601, and a resist pattern 900 is formed on the sensor film 600. With the resist pattern 900 as a mask, the sensor film 600 is milled and patterned. As shown in FIGS. 22A and 22B (FIG. 22B is a cross sectional view seen from the arrow α4-α4 in FIG. 22A), an insulation layer 602 (e.g., Al2O3) and a non-magnetic metal layer 603 (e.g., Cr or CrTi) are filled in the milled indented parts. The resist is then lifted off from the sensor film 600, and the structure becomes as shown in FIGS. 23A and 23B (FIG. 23B is a cross sectional view seen from the arrow α5-α5 in FIG. 23A). The formation shown in FIG. 23A continues in the depth direction of the figure.
Next, as shown in FIG. 24A, FIG. 24B (FIG. 24B is a cross sectional view seen from the arrow α6-α6 in FIG. 24A) and FIG. 24C (FIG. 24C is a cross sectional view seen from the arrow α7-α7 in FIG. 24A), a resist pattern 910 is formed to form a bias layer on the rear part. With the resist pattern 910 as a mask, the rear part is milled, and a bias layer 800 is filled in the milled indented part. That is, the hard magnet layer 800 is formed on an insulation layer 710 (e.g., Al2O3) that is laid down (e.g., forming a CoPt layer 800 as a hard magnet layer on a Cr base layer). After a cap layer 810 (e.g., Cr) is disposed on the layer 800, the resist pattern 910 is lifted off.
In the production method based on the conventional multilayer structure as discussed above, when milling the rear part to lay down the above-discussed hard magnet layer 800, because the milling speed(s) is not identical for non-magnetic layer 603 and the sensor film 600, which is the DFL element part, the depths of milling differ. As a result, the hard magnet layer 800 filled in the milled section does not become flat. In other words, the hard magnet layer 800 does not become flat in the X direction as shown in FIG. 20, but rather includes a U-shaped formation 800a, in which a part of the hard magnet layer 800 corresponding to the DFL element part (sensor film 600) is lowered.
This causes a chance that the bias magnetic field from the hard magnet layer 800 is not effectively intensively applied to the DFL element.
In addition, as shown in FIG. 25, after forming the hard magnet layer 800 on the rear part, ion beam etching is performed to clean the entire upper formation surface prior to forming an upper side exchange-coupling shield layer (shield layer). The flatness is maintained if the ion beam etching rates are the same for, for example, the cap layer (NiFe) that is the top layer at the DFL element part (sensor film 600) and the non-magnetic metal layer 603. However, if the ion beam etching is faster for the non-magnetic metal layer 603, then the flatness of the non-magnetic metal layer 603 decreases (for instance, a depressed portion is formed on the non-magnetic metal layer 603). FIG. 26 illustrates the shield layer 605 that was formed after the ion beam etching.
The present invention was made in consideration of these facts. The present invention has an objection to provide a magnetoresistive effect element, in which flatness of an orthogonalizing bias function part (a hard magnet layer) positioned at the rear part of the MR element having two free layers can be maintained, and in which unnecessary magnetic flux is prevented from leaking to the upper shield, thereby improving fluctuation of a QST (quasi-static test) waveform.